Research and development are underway to produce the next generation of accelerators for accelerating charged particles to extremely high energies for high energy physics. In such accelerators, a beam of charged particles is generally caused to travel around a circular path. In very large accelerators, the circular path may be many kilometers (km) in circumference. Such accelerators require high-field, small-bore, dipole beam bending magnets capable of producing magnetic fields on the order of ten T (Tesla). In such accelerators, the magnets must produce extremely high intensity magnetic fields to minimize the circumference of the circular beam tube, so as to minimize the cost of the accelerator. A very large accelerator is extremely costly, because of the high costs of land acquisition, tunnel construction, shielding construction, and the provision of the cryogenic distribution system, vacuum system and the like. Even with magnets developing fields of ten T, an accelerator capable of developing an energy level of 30 TeV could have a circumference as great as 80 kilometers.
The cost of the magnets for such an accelerator can be reduced by reducing the size of the magnets. The particle beam for such an accelerator can in principle be as small as 20 millimeters (mm) in diameter, or even smaller. The beam can be contained within an accelerator bore having a diameter of about 40 mm, for which a winding diameter of about 60 mm is feasible. However, it is difficult to make coils having such a small winding diameter. Moreover, such coils require higher current densities in the superconductor than is used in present accelerator magnets.
The present invention relates to the production of such coils of the layer or shell type which are curved and generally pancake-shaped and are adapted to be mounted around a 180-degree segment of a cylindrical support. Such curved layer-type coils are used in pairs on opposite sides of the cylindrical support. The complete coil assembly may have several superimposed layers on both sides of the cylindrical support. In such layer or shell-type windings, the electrical conductor or cable is generally rectangular or flat in cross section and is wound with the edge of the cable engaging the cylindrical supporting surface. Such coils are difficult to wind because of the curvature of the coils. To accommodate the curvature, the conductors must be wedge-shaped in cross section, or may employ separate wedges, interspersed between some of the turns, to make the winding solid and immovable, so that the winding will withstand the very high magnetic forces which are developed in the coils, due to the very high currents which are possible under superconductive operating conditions. Any movement of the coil conductors is to be avoided during operation, because such movement generates heat which can cause the loss of superconductivity.
In such curved shell-type coils, the turns are generally oval-shaped, with longitudinal portions and curved portions extending therebetween. Even though the turns are wound under tension, the longitudinal portions tend to drape along catenary curves. This draping effect makes it difficult to confine each coil to a 180-degree segment of a cylindrical supporting surface. The draping of the turns tends to make the coil less solid and compact than would be desirable.
In the past, attempts have been made to deal with the problem of draping and looseness of the turns by clamping the finished coils in various ways, such as by the use of rings and helical wrappings. Moreover, adhesive materials have been used in significant quantities to fill in the spaces between the turns and to solidify coils. Such adhesive materials have included epoxy resins which are polymerized after the coils have been completed.
The principal object of the present invention is to provide an improved method and apparatus for making such coils, so as to deal more effectively with the problems of draping and looseness of the turns, whereby coils of enhanced quality may be produced. The improved coils are more solid and compact and less subject to movement during operation, so that the loss of superconductivity is less likely to occur. When superconductivity is lost, it is necessary to again cool the coils until they become superconductive. The accelerator goes out of service when superconductivity is lost, and can only be returned to service when superconductivity has been restored in all magnets.